When a strong electric field (E) such as laser beam is applied to a substance, the substance shows polarization (P) which is expressed by the following general equation: EQU P=X.sup.(1) E+X.sup.(2) EE+X.sup.(3) EEE+... ...
(wherein X.sup.(1) means linear optical susceptibility and X.sup.(n) (n is an integer of not less than 2) means nonlinear optical susceptibility). The nonlinear optical effects are those expressed by the high order terms of E, i.e., terms of E of not less than square.
The effects expressed by the square term are called second-order nonlinear optical effects. Examples of the second-order nonlinear optical effects include frequency conversion effects such as second harmonic generation (SHG) or parametric oscillation, and linear electro-optic effect (Pockels effect). By utilizing these effects, second-order nonlinear optical devices which are industrially important, such as frequency converters including second harmonic generators (SHG devices) and parametric oscillators, or electro-optic devices such as optical switches and optical modulators can be obtained.
The second-order nonlinear optical devices include at least one optical element of an optical medium which should have a substantially optically smooth surface to be impinged, transmitted and/or propagated by laser beam or the like. In case of electro-optic devices, the devices further comprise electrodes for applying electric field.
Employment of an optical medium which shows higher performance or higher nonlinear optical effects for constituting the second-order nonlinear optical device is advantageous because (i}the power of the light source may be reduced, (ii) the size of the device may be made small, (iii) the voltage of the applied electric field may be reduced and (iv) the price of the device may be made less expensive.
The second-order nonlinear optical media have strong anisotropy with respect to the expression of the effects (i.e., the anisotropic dependency on the optical field and external electric field). Therefore, a device structure, namely, an element structure for allowing the optical medium to efficiently exhibit the effects is required.
Conventional nonlinear optical media include
(i) inorganic ferroelectric crystals such as lithium niobate (LN) or potassium dihydrogen phosphate (KDP) crystals; PA1 (ii) organic crystals such as 2-methyl-4-nitroaniline (MNA); and PA1 (iii) organic solid solution of polymers and organic molecules which can manifest second-order nonlinear optical effects (e.g., poled polymers hereinbelow described).
The nonlinear optical media (i) were firstly developed in the art and their processing technologies for an optical element or device are best known. However, their second-order nonlinear optical effects are not large. Thus, the performance of the second-order nonlinear optical devices utilizing the nonlinear optical media (i) is unsatisfactory. They are large in size and expensive. In addition, the LN crystals which show the best performance of the media (i) can be damaged by light, which is a serious problem in practical industrial applications.
The nonlinear optical media (ii) are receiving much attention recently as optical media superior to the media (i) because of the large optical nonlinearity of organic molecules due to the intramolecular x electronic fluctuation, fast response and high resistance against laser beam.
For example, 2-methyl-4-nitroaniline (MNA) crystal was reported to have the highest nonlinear optical effects among the optical media (ii), which are larger than those exhibited by the LN crystal which is an inorganic ferroelectric nonlinear optical crystal (e.g., J. Appl. Phys., 50(4), 2523 (1979); J. Chem. Phys., 75(3), 1509 (1981)).
However, the nonlinear optical effects of MNA crystal are not so strikingly larger than those exhibited by LN crystal. Further, MNA crystal has practical problems in that it is water-soluble and sublimated at room temperature.
The nonlinear optical media (iii) have been developed because of the good processability of the polymers. However, their second-order nonlinear optical effects are much smaller than those typical of the optical media (ii), and at present, even smaller than those of LN crystal. This is because the density of the component (i.e., the organic molecule) exhibiting the nonlinear optical effects is lowered by the existence of the polymer and the degree of orientation of the component (pigment) exhibiting the nonlinear optical effects cannot be made so high by the poling treatment (poling treatment is detailed in Proceedings of MRS Conference, vol. 109, "Nonlinear Optical Properties of Polymers", Ed. by A. J. Heeger et al., 1988, p19). Further, the degree of orientation of the component providing the nonlinear optical effects decreases with time because of orientation relaxation, so that the performance thereof decreases with time accordingly.
Thus, all of the conventional media (i), (ii) and (iii) is unsatisfactory for realization of a second-order nonlinear optical device with high performance.